U.S. patent application number 10/201631 was filed with the patent office on 2003-03-27 for loudspeaker.
This patent application is currently assigned to NEW TRANSDUCERS LIMITED. Invention is credited to Bank, Graham, Cassey, Martin Christopher, Colloms, Martin, Harris, Neil, Owen, Neil Simon.
Application Number | 20030059069 10/201631 |
Document ID | / |
Family ID | 27546624 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030059069 |
Kind Code |
A1 |
Bank, Graham ; et
al. |
March 27, 2003 |
Loudspeaker
Abstract
A bending wave loudspeaker includes a transparent acoustic
radiator capable of supporting bending wave vibration and an
electromechanical force transducer mounted to the acoustic radiator
to excite bending waves in the acoustic radiator to produce an
acoustic output. The transducer has an intended operative frequency
range and includes a resonant element having a frequency
distribution of modes in the operative frequency range and a
coupler for mounting the transducer to the acoustic radiator. The
loudspeaker may be incorporated in a telephone handset or a visual
display unit.
Inventors: |
Bank, Graham; (Woodbridge,
GB) ; Colloms, Martin; (London, GB) ; Owen,
Neil Simon; (Huntingdon, GB) ; Harris, Neil;
(Cambridge, GB) ; Cassey, Martin Christopher;
(Cambridge, GB) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NEW TRANSDUCERS LIMITED
|
Family ID: |
27546624 |
Appl. No.: |
10/201631 |
Filed: |
July 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10201631 |
Jul 24, 2002 |
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09768002 |
Jan 24, 2001 |
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60309792 |
Aug 6, 2001 |
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60178315 |
Jan 27, 2000 |
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60205465 |
May 19, 2000 |
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60218062 |
Jul 13, 2000 |
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Current U.S.
Class: |
381/152 ;
381/190; 381/191; 381/431 |
Current CPC
Class: |
H04R 7/045 20130101;
H04R 2499/13 20130101; H04R 17/00 20130101 |
Class at
Publication: |
381/152 ;
381/190; 381/191; 381/431 |
International
Class: |
H04R 025/00; H04R
001/00; H04R 009/06; H04R 011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 1, 2001 |
GB |
0118750.9 |
Claims
We claim:
1. A bending wave loudspeaker comprising: a transparent acoustic
radiator adapted to support bending wave vibration; and an
electromechanical force transducer mounted to the acoustic radiator
to excite bending waves in the acoustic radiator to produce an
acoustic output, wherein the transducer has an intended operative
frequency range and comprises: at least one resonant element having
a frequency distribution of modes in the operative frequency range;
and a coupler for mounting the transducer to the acoustic
radiator.
2. A loudspeaker according to claim 1, wherein parameters of the
resonant element are selected to enhance the distribution of modes
in the resonant element in the operative frequency range.
3. A loudspeaker according to claim 2, wherein the distribution of
modes in the resonant element has a density of modes which is
sufficient for the resonant element to provide an effective mean
average force which is substantially constant with frequency.
4. A loudspeaker according to claim 2, wherein the modes are
distributed substantially evenly over the intended operative
frequency range.
5. A loudspeaker according to claim 1, wherein the resonant element
is modal along two substantially normal axes, each axis having an
associated fundamental frequency, and wherein the ratio of the two
associated fundamental frequencies is adjusted for best modal
distribution.
6. A loudspeaker according to claim 5, wherein the ratio of the two
fundamental frequencies is about 9:7.
7. A loudspeaker according to claim 1, wherein the transducer
comprises a plurality of resonant elements each having a
distribution of modes, wherein the modes of the resonant elements
are arranged to interleave in the operative frequency range whereby
the distribution of modes in the transducer is enhanced.
8. A loudspeaker according to claim 1, wherein the resonant element
is plate-like.
9. A loudspeaker according to claim 1, wherein the shape of the
resonant element is selected from the group consisting of
beam-like, trapezoidal, hyperelliptical, generally disc shaped, and
rectangular.
10. A loudspeaker according to claim 9, wherein the resonant
element is plate-like.
11. A loudspeaker according to claim 1, wherein the acoustic
radiator has a first and a second face, the transducer being
mounted to a first face of the acoustic radiator and a mask being
mounted to the second face of the acoustic radiator to obscure the
transducer.
12. A loudspeaker according to claim 11, wherein parameters of the
resonant element are selected to enhance the distribution of modes
in the resonant element in the operative frequency range.
13. A loudspeaker according to claim 12, wherein the distribution
of modes in the resonant element has a density of modes which is
sufficient for the resonant element to provide an effective mean
average force which is substantially constant with frequency.
14. A loudspeaker according to claim 12, wherein the modes are
distributed substantially evenly over the intended operative
frequency range.
15. A loudspeaker according to claim 12, wherein the resonant
element is modal along two substantially normal axes, wherein each
axis has an associated fundamental frequency, and wherein the ratio
of the two associated fundamental frequencies is adjusted for best
modal distribution.
16. A loudspeaker according to claim 13, further comprising: a
frame which at least partially surrounds the acoustic radiator; and
a suspension for mounting the acoustic radiator to the frame.
17. A loudspeaker according to claim 16, wherein the frame acts as
a baffle.
18. A loudspeaker according to claim 16, wherein parameters of the
resonant element are selected to enhance the distribution of modes
in the resonant element in the operative frequency range.
19. A loudspeaker according to claim 18, wherein the distribution
of modes in the resonant element has a density of modes which is
sufficient for the resonant element to provide an effective mean
average force which is substantially constant with frequency.
20. A loudspeaker according to claim 18, wherein the modes are
distributed substantially evenly over the intended operative
frequency range.
21. A telephone handset comprising: a body supporting a microphone,
at least one key, a display, and a window mounted over the display;
and a bending wave loudspeaker comprising: a transparent acoustic
radiator adapted to support bending wave vibration; and an
electromechanical force transducer mounted to the acoustic radiator
to excite bending waves in the acoustic radiator to produce an
acoustic output, wherein the transducer has an intended operative
frequency range and comprises: a resonant element having a
frequency distribution of modes in the operative frequency range;
and a coupler for mounting the transducer to the acoustic radiator,
wherein the window is operable as the acoustic radiator.
22. A telephone handset according to claim 21, wherein parameters
of the resonant element are selected to enhance the distribution of
modes in the resonant element in the operative frequency range.
23. A telephone handset according to claim 22, wherein the
distribution of modes in the resonant element has a density of
modes which is sufficient for the resonant element to provide an
effective mean average force which is substantially constant with
frequency.
24. A telephone handset according to claim 22, wherein the modes
are distributed substantially evenly over the intended operative
frequency range.
25. A telephone handset according to claim 21, further comprising:
a suspension which supports the window on the body and which
prevents transmission of vibration from the window to the body.
26. A visual display unit comprising: a body supporting a display
unit and a window mounted over the display; and a bending wave
loudspeaker comprising: a transparent acoustic radiator capable of
supporting bending wave vibration; and an electromechanical force
transducer mounted to the acoustic radiator to excite bending waves
in the acoustic radiator to produce an acoustic output, wherein the
transducer has an intended operative frequency range and comprises:
a resonant element having a frequency distribution of modes in the
operative frequency range; and a coupler for mounting the
transducer to the acoustic radiator, wherein the window is operable
as the acoustic radiator.
27. A visual display unit according to claim 26, wherein parameters
of the resonant element are selected to enhance the distribution of
modes in the resonant element in the operative frequency range.
28. A visual display unit according to claim 27, wherein the
distribution of modes in the resonant element has a density of
modes which is sufficient for the resonant element to provide an
effective mean average force which is substantially constant with
frequency.
29. A visual display unit according to claim 27, wherein the modes
are distributed substantially evenly over the intended operative
frequency range.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Serial No. 60/309,792, filed Aug. 6, 2001 (incorporated
by reference in its entirety), and is a continuation-in-part
application of U.S. patent application Ser. No. 09/768,002 filed
Jan. 24, 2001, which claims the benefit of U.S. Provisional
Application Serial No. 60/178,315, filed Jan. 27, 2000, No.
60/205,465, filed May 19, 2000, and No. 60/218,062, filed Jul. 13,
2000.
BACKGROUND
[0002] This invention relates to a bending wave panel speaker,
particularly but not exclusively, bending wave panel speakers known
as distributed mode loudspeakers, e.g., as taught in WO 97/09842
and corresponding U.S. Pat. No. 6,332,029, the latter of which is
herein incorporated by reference.
[0003] It is known from WO 97/09842 (U.S. Pat. No. 6,332,029) and
other publications (e.g. WO97/09846 (U.S. patent application Ser.
No. 09/029,360), WO99/08479 (U.S. patent application Ser. No.
09/497,655) and WO00/33612 (U.S. patent application Ser. No.
09/450,754)) in the name of New Transducers Limited to apply one or
more exciters to a bending wave panel for energising bending waves
in the panel. The locations of the exciters may be chosen with
consideration for modal drive coupling, moderating directional
effects or adjusting behaviour through the coincidence frequency
region.
SUMMARY OF THE INVENTION
[0004] According to the invention, there is provided a bending wave
loudspeaker comprising an acoustic radiator capable of supporting
bending wave vibration and an electromechanical force transducer
mounted to the acoustic radiator to excite bending wave vibration
in the acoustic radiator to produce an acoustic output, the
transducer having an intended operative frequency range and
comprising a resonant element having a frequency distribution of
modes in the operative frequency range and a coupler or coupling
means on the resonant element for mounting the transducer to the
acoustic radiator, wherein the acoustic radiator is
transparent.
[0005] The loudspeaker may further comprise a mask which obscures
the transducer. The loudspeaker may be suspended in a frame, which
may be open or closed. The frame may be adapted for mounting in
another structure.
[0006] The resonant element may be active, e.g., it may be a
piezoelectric transducer and it may be in the form of a strip of
piezoelectric material. Alternatively, the resonant element may be
passive and the transducer may further comprise an active
transducer, e.g., an inertial or grounded vibration transducer,
actuator or exciter, e.g., a moving coil transducer. The active
transducer may be a bending or torsional transducer (e.g. of the
type taught in WO00/13464 (U.S. patent application Ser. No.
09/384,419)). Furthermore, the transducer may comprise a
combination of passive and active elements to form a hybrid
transducer.
[0007] A number of transducer, exciter, or actuator mechanisms have
been developed to apply a force to a structure such as an acoustic
radiator of a loudspeaker. There are various types of these
transducer mechanisms, for example moving coil, moving magnet,
piezoelectric, or magnetostrictive types. Typically, electrodynamic
speakers using coil and magnet type transducers lose 99% of their
input energy to heat whereas a piezoelectric transducer may lose as
little as 1%. Thus, piezoelectric transducers are popular because
of their high efficiency.
[0008] There are several problems with piezoelectric transducers,
for example, they are inherently very stiff, for example comparable
to brass foil, and are, therefore, thus difficult to match to an
acoustic radiator, especially to the air. Raising the stiffness of
the transducer moves the fundamental resonant mode to a higher
frequency. Thus, such piezoelectric transducers may be considered
to have two operating ranges. The first operating range is below
the fundamental resonance of the transducer. This is the "stiffness
controlled" range where velocity rises with frequency and the
output response usually needs equalisation. This leads to a loss in
available efficiency. The second range is the resonance range
beyond the stiffness range, which is generally avoided because the
resonances are rather fierce.
[0009] Moreover, the general teaching is to suppress resonances in
a transducer. Thus, piezoelectric transducers are generally used
only used in the frequency range below or at the fundamental
resonance of the transducers. Where piezoelectric transducers are
used above the fundamental resonance frequency it is necessary to
apply damping to suppress resonance peaks.
[0010] The problems associated with piezoelectric transducers
similarly apply to transducers comprising other "smart" materials,
i.e., magnetostrictive, electrostrictive, and electret type
materials. Various piezoelectric transducers are also known, for
example as described in EP 0993 231A of Shinsei Corporation, EP
0881 856A of Shinsei Corporation, U.S. Pat. No. 4,593,160 of Murata
Manufacturing Co. Limited, U.S. Pat. No. 4,401,857 of Sanyo
Electric Co. Limited, U.S. Pat. No. 4,481,663 of Altec Corporation
and UK patent application GB2,166,022A of Sawafuji. However, it is
an object of the invention to employ an improved transducer.
[0011] The transducer used in the present invention may be
considered to be an intendedly modal transducer. The coupler may be
attached to the resonant element at a position which is beneficial
for coupling modal activity of the resonant element to the
interface. The parameters (e.g., aspect ratio, bending stiffness,
thickness and geometry) of the resonant element may be selected to
enhance the distribution of modes in the resonant element in the
operative frequency range. The bending stiffness and thickness of
the resonant element may be selected to be isotropic or
anisotropic. The variation of bending stiffness and/or thickness
may be selected to enhance the distribution of modes in the
resonant element. Analysis (e.g., computer simulation using FEA or
modelling) may be used to select the parameters.
[0012] The distribution may be enhanced by ensuring a first mode of
the active element is near to the lowest operating frequency of
interest. The distribution may also be enhanced by ensuring a
satisfactory, e.g. high, density of modes in the operative
frequency range. The density of modes is preferably sufficient for
the active element to provide an effective mean average force which
is substantially constant with frequency. Good energy transfer may
provide beneficial smoothing of modal resonances. Alternatively, or
additionally, the distribution of modes may be enhanced by
distributing the resonant bending wave modes substantially evenly
in frequency, i.e. to smooth peaks in the frequency response caused
by "bunching" or clustering of the modes. Such a transducer may
thus be known as a distributed mode transducer or DMT.
[0013] Such an intendedly modal or distributed mode transducer is
described in International patent application WO01/54450 and U.S.
patent application Ser. No. 09/768,002, filed Jan. 24, 2001 (the
latter of which is herein incorporated by reference in its
entirety).
[0014] The transducer may comprise a plurality of resonant elements
each having a distribution of modes, the modes of the resonant
elements arranged to interleave in the operative frequency range
and enhance the distribution of modes in the transducer. The
resonant elements may have different fundamental frequencies and
thus, the parameters (e.g., loading, geometry or bending stiffness)
of the resonant elements may be different.
[0015] The resonant elements may be coupled together by a connector
or connecting means in any convenient way, e.g. on generally stiff
stubs, between the elements. The resonant elements are preferably
coupled at coupling points which enhance the modality of the
transducer and/or enhance the coupling at the site to which the
force is to be applied. Parameters of the connecting means may be
selected to enhance the modal distribution in the resonant element.
The resonant elements may be arranged in a stack. The coupling
points may be axially aligned.
[0016] The resonant element may be plate-like or may be curved out
of planar. A plate-like resonant element may be formed with slots
or discontinuities to form a multi-resonant system. The resonant
element may be beam-shaped, trapezoidal, hyperelliptical, or may be
generally disc shaped. Alternatively, the resonant element may be
rectangular and may be curved out of the plane of the rectangle
about an axis along the short axis of symmetry.
[0017] The resonant element may be modal along two substantially
normal axes, each axis having an associated fundamental frequency.
The ratio of the two fundamental frequencies may be adjusted for
best modal distribution, e.g., about 9:7 (.about.1.286:1).
[0018] As examples, the arrangement of such a modal transducer may
be any of: a flat piezoelectric disc; a combination of at least two
or preferably at least three flat piezoelectric discs; two
coincident piezoelectric beams; a combination of multiple
coincident piezoelectric beams; a curved piezoelectric plate; a
combination of multiple curved piezoelectric plates or two
coincident curved piezoelectric beams.
[0019] The interleaving of the distribution of the modes in each
resonant element may be enhanced by optimising the frequency ratio
of the resonant elements, namely the ratio of the frequencies of
each fundamental resonance of each resonant element. Thus, the
parameter of each resonant element relative to one another may be
altered to enhance the overall modal distribution of the
transducer.
[0020] When using two active resonant elements in the form of
beams, the two beams may have a frequency ratio (i.e., ratio of
fundamental frequency) of about 1.27:1. For a transducer comprising
three beams, the frequency ratio may be about 1.315:1.147:1. For a
transducer comprising two discs, the frequency ratio may be about
1.1+/-0.02 to 1 to optimise high order modal density or may be
about about 3.2 to 1 to optimise low order modal density. For a
transducer comprising three discs, the frequency ratio may be about
3.03:1.63:1 or may be about 8.19:3.20:1.
[0021] The parameters of the coupler may be selected to enhance the
distribution of modes in the resonant element in the operative
frequency range. The coupler may be vestigial, e.g., a controlled
layer of adhesive.
[0022] The coupler may be positioned asymmetrically with respect to
the panel so that the transducer is coupled asymmetrically. The
asymmetry may be achieved in several ways, for example by adjusting
the position or orientation of the transducer with respect to axes
of symmetry in the panel or the transducer.
[0023] The coupler may form a line of attachment. Alternatively,
the coupler may form a point or small local area of attachment
where the area of attachment is small in relation to the size of
the resonant element. The coupler may be in the form of a stub and
have a small diameter, e.g., about 3 to 4 mm. The coupler may be
low mass.
[0024] The coupler may comprise more than one coupling point and
may comprise a combination of points and/or lines of attachment.
For example, two points or small local areas of attachment may be
used, one positioned near centre and one positioned at the edge of
the active element. This may be useful for plate-like transducers
which are generally stiff and have high natural resonance
frequencies.
[0025] Alternatively only a single coupling point may be provided.
This may provide the benefit, in the case of a multi-resonant
element array, that the output of all the resonant elements is
summed through the single coupler so that it is not necessary for
the output to be summed by the load. The coupler may be chosen to
be located at an anti-node on the resonant element and may be
chosen to deliver a constant average force with frequency. The
coupler may be positioned away from the centre of the resonant
element.
[0026] The position and/or the orientation of the line of
attachment may be chosen to optimise the modal density of the
resonant element. The line of attachment is preferably not
coincident with a line of symmetry of the resonant element. For
example, for a rectangular resonant element, the line of attachment
may be offset from the short axis of symmetry (or centre line) of
the resonant element. The line of attachment may have an
orientation which is not parallel to a symmetry axis of the
panel.
[0027] The shape of the resonant element may be selected to provide
an off-centre line of attachment which is generally at the centre
of mass of the resonant element. One advantage of this embodiment
is that the transducer is attached at its centre of mass and thus
there is no inertial imbalance. This may be achieved by an
asymmetric shaped resonant element which may be in the shape of a
trapezium or trapezoid.
[0028] For a transducer comprising a beam-like or generally
rectangular resonant element, the line of attachment may extend
across the width of the resonant element. The area of the resonant
element may be small relative to that of the acoustic radiator.
[0029] The acoustic radiator may be in the form of a panel. The
panel may be flat and may be lightweight. The material of the
acoustic radiator may be anisotropic or isotropic.
[0030] The acoustic radiator may have a distribution of resonant
bending wave modes and may produce an acoustic output when the
modes are excited by the transducer. The properties of the acoustic
radiator may be chosen to distribute the resonant bending wave
modes substantially evenly in frequency (i.e., to smooth peaks in
the frequency response caused by "bunching" or clustering of the
modes).
[0031] In particular, the properties of the acoustic radiator may
be chosen to distribute the lower frequency resonant bending wave
modes substantially evenly in frequency. The lower frequency
resonant bending wave modes are preferably the ten to twenty lowest
frequency resonant bending wave modes of the acoustic radiator.
[0032] The parameters of the transducer may be selected to match
the mechanical properties of the transducer to those of the
acoustic radiator. By matching the source (transducer) and load
(acoustic radiator) mechanical impedances, mechanical power may be
transmitted with high efficiency.
[0033] The transducer location may be chosen to couple
substantially evenly to the resonant bending wave modes in the
acoustic radiator, in particular to lower frequency resonant
bending wave modes. In other words, the transducer may be mounted
at a location where the number of vibrationally active resonance
anti-nodes in the acoustic radiator is relatively high and
conversely the number of resonance nodes is relatively low. Any
such location may be used, but the most convenient locations are
the near-central locations between about 38% to about 62% along
each of the length and width axes of the acoustic radiator, but
off-centre. Specific or preferential locations are at about
{fraction (3/7)}, about {fraction (4/9)} or about {fraction (5/13)}
of the distance along the axes; a different ratio for the length
axis and the width axis is preferred. Preferred is about {fraction
(4/9)} length and about {fraction (3/7)} width of an isotropic
panel having an aspect ratio of about 1:1.13 or about 1:1.41.
[0034] Alternatively, the transducers may be mounted to an edge or
marginal portion of the acoustic radiator, e.g. as taught in
International application WO00/02417 and U.S. patent application
Ser. No. 09/752,830, the latter of which is herein incorporated by
reference. The edge or marginal portion of the acoustic radiator
may be clamped to improve acoustic performance as taught in
WO99/37121 and U.S. patent application Ser. No. 09/233,037, the
latter of which is herein incorporated by reference.
[0035] The operative frequency range may be over a relatively broad
frequency range and may be in the audio range and/or ultrasonic
range. There may also be applications for sonar and sound ranging
and imaging where a wider bandwidth and/or higher possible power
will be useful by virtue of distributed mode transducer operation.
Thus, operation over a range greater than the range defined by a
single dominant, natural resonance of the transducer may be
achieved.
[0036] The lowest frequency in the operative frequency range is
preferably above a predetermined lower limit which is about the
fundamental resonance of the transducer.
[0037] For example, for a beam-like active resonant element, the
force may be taken from the centre of the beam, and may be matched
to the mode shape in the acoustic radiator to which it is attached.
In this way, the action and reaction may co-operate to give a
constant output with frequency. By connecting the resonant element
to the acoustic radiator, at an anti-node of the resonant element,
the first resonance of the resonant element may appear to be a low
impedance. In this way, the acoustic radiator should not amplify
the resonance of the resonant element.
[0038] According to a second embodiment of the invention, there is
provided a telephone handset, e.g. for a mobile phone or wireless
telephone, comprising a body supporting a microphone, keys, a
display, and a window mounted over the display. The handset further
comprises a loudspeaker as described above and the window acts as
the acoustic radiator of the loudspeaker.
[0039] The window may be supported on the body via a suspension
whereby vibration from the window is prevented from being
transmitted by the body to the microphone.
[0040] According to a third embodiment of the invention, there is
provided a visual display unit, e.g. a television, comprising a
body supporting a display unit, e.g. LCD or TFT display unit, and a
window mounted over the display. The visual display unit further
comprises a loudspeaker as described above and the window acts as
the acoustic radiator of the loudspeaker.
BRIEF DESCRIPTION OF DRAWINGS
[0041] Examples that embody the best mode for carrying out the
invention are described in detail below and are diagrammatically
illustrated in the accompanying drawings in which:
[0042] FIG. 1 shows a perspective view of a handset embodying the
present invention;
[0043] FIG. 2 shows a cross-sectional view taken along line AA of
FIG. 1;
[0044] FIG. 3 shows a front view of a loudspeaker embodying the
present invention;
[0045] FIG. 4 is a cross-sectional view of a loudspeaker taken
along line AA of FIG. 3 mounted in a frame;
[0046] FIGS. 5 to 11 are side views of alternative modal
transducers which may be used in the present invention;
[0047] FIG. 12 is a plan view of an alternative modal transducer
which may be used in the present invention;
[0048] FIG. 13A is a schematic plan view of a parameterised model
of a transducer which may be used in the present invention;
[0049] FIG. 13B is a section perpendicular to the line of
attachment of the transducer of FIG. 13A;
[0050] FIG. 14A is a schematic plan view of a parameterised model
of a transducer which may be used in the present invention; and
[0051] FIG. 14B is a second schematic plan view of the transducer
of FIG. 14A.
DETAILED DESCRIPTION
[0052] FIGS. 1 and 2 show a telephone handset (58) which may be in
the form of a mobile phone, wireless telephone handset, or handset
connected to a landline. The handset (58) comprises a back part
(60) and a front part (62) which carries the standard components,
namely a microphone (64), keys (65) and a display window (66)
fitted with an opaque surround (68). The display window (66) is
fitted above a display (108) which may be a liquid crystal display
(LCD) or thin film transistor (TFT) display. The display (108) is
supported on the front part (62) by a suspension (110), which is
fitted around the periphery of the display (108).
[0053] The display window (66) is in the form of a panel which is
designed to be capable of supporting bending waves, particularly
resonant bending wave modes as taught in WO97/09842 (U.S. Pat. No.
6,332,029) and WO97/09854 (U.S. patent application Ser. No.
09/029,059) of the present applicant. A transducer (86) is mounted
to the display window (66) to launch or to excite bending wave
vibration to produce an acoustic output. The transducer (86) is an
intendedly modal transducer or distributed mode transducer as
hereinbefore described and as described in WO01/54450 and in U.S.
patent application Ser. No. 09/768,002.
[0054] The transducer (86) comprises upper and lower bimorph beams
(90, 88), the upper beam (90) being connected to the display window
(66) by a stub (92) which extends across the width of the beams.
The stub (92) may be about 1-2 mm wide and high and may be made
from hard plastics and/or metal with suitable insulating layers to
prevent electrical short circuits. The beams (90, 88) are of
transparent material (i.e., PZLT material) used with thin film
electrodes. Thus, the transducer (86) is substantially transparent
although there may be a minor visual obstruction caused by the
stubs.
[0055] The beams (90, 88) are of unequal lengths; the upper beam
(90) is longer than the lower beam (88). Each beam (90, 88) can
consist of three layers, namely two outer layers of piezoelectric
ceramic material, e.g. PZT 5H, sandwiching a central brass vane
layer. The outer layers may be attached to the brass vane layer by
adhesive layers which are typically about 10-15 microns thick.
[0056] The display window (66) is mounted into the front part (62)
by way of a suspension (84) which extends around the periphery of
the window. The suspension (84) sets the boundary condition for the
display window (66) and may be used to prevent structure borne
vibration from being transmitted from the window (66) back to the
microphone (64).
[0057] FIG. 3 shows a loudspeaker (154) which comprises a panel
(67) which is designed to be capable of supporting bending waves,
particularly resonant bending wave modes. The panel (67) is made
from a transparent material, e.g. glass. A transducer (not shown)
is mounted near an edge of the panel to excite it to produce
vibration to produce an acoustic output. A mask (152) is mounted in
front of the edges of the panel (67) to obscure the transducer. The
panel (67) is suspended in a frame (156), whereby the loudspeaker
may be adapted for mounting in any location.
[0058] FIG. 4 shows an application of the loudspeaker of FIG. 3.
The loudspeaker forms a window panel for a display (108) which is
supported on the frame (156) by a flexible front suspension (170)
which extends around the periphery of the display. The loudspeaker
is supported in the frame (156) by a flexible rear suspension (172)
which extends around the periphery of the panel (67).
[0059] The panel (67) is driven by an intendedly modal transducer
(158) by way of a stub (92). The transducer (158) is in the form of
a piezoelectric plate which is driven by an input through
connection leads. The transducer (158) is obscured from a viewer by
the mask (152) which may be printed onto the front or back surface
of the panel (67).
[0060] The remaining figures show alternative transducers which may
be used in conjunction with the loudspeaker applications embodied
in FIGS. 1 to 4. Each transducer is capable of being mounted to a
transparent panel or other load device. An intendedly modal
transducer may be designed with reduced mass and depth compared to
a moving coil/permanent magnet design. Accordingly, the use of such
a transducer should reduce the overall weight of the loudspeaker
and the transducer should be suitable for installations in which
space is limited, e.g. in phone handsets. For example, a standard
moving coil electromagnetic transducer generally has a weight of
approximately 30 g and a height of approximately 13 mm. In
contrast, a two-beam modal transducer may have a weight of only
approximately 2 g and a height of approximately 5 mm.
[0061] FIG. 5 shows a transducer (42) which comprises a first
piezoelectric beam (43) on the back of which is mounted a second
piezoelectric beam (51) by connecting means in the form of a stub
(48) located at the centre of both beams (43, 51). Each beam (43,
51) is a bi-morph. The first beam (43) comprises two layers (44,46)
of piezoelectric material and the second beam (51) comprises two
layers (50,52). The poling directions of each layer of
piezoelectric material are shown by arrows (49). Each layer (44,
50) has an opposite poling direction to the other layers (46, 52),
respectively, in the bi-morph. The bimorph may also comprise a
central conducting vane which allows a parallel electrical
connection as well as adding a strengthening component to the
ceramic piezoelectric layers. Each layer of each beam (43, 51) may
be made of the same/different piezoelectric material. Each layer is
generally of a different length.
[0062] The first piezoelectric beam (43) is mounted on a panel (54)
by a coupler or coupling means in the form of a stub (56) located
at the centre of the first beam. By mounting the first beam (43) at
its centre only the even order modes will produce output. By
locating the second beam (51) behind the first beam (43), and
coupling both beams (43, 51) centrally by way of a stub (48) they
can both be considered to be driving the same axially aligned or
co-incident position.
[0063] When the beams are joined together, the resulting
distribution of modes is not the sum of the separate sets of
frequencies, because each beam modifies the modes of the other. The
two beams are designed so that their individual modal distributions
are interleaved to enhance the overall modality of the transducer.
The two beams add together to produce a useable output over a
frequency range of interest. Local narrow dips occur because of the
interaction between the piezoelectric beams at their individual
even order modes.
[0064] The second beam may be chosen by using the ratio of the
fundamental resonance of the two beams. If the materials and
thicknesses are identical, then the ratio of frequencies is just
the square of the ratio of lengths. If the higher f0 (fundamental
frequency) is simply placed half way between f0 and f1 of the
other, larger beam, f3 of the smaller beam and f4 of the lower beam
coincide.
[0065] Plotting a graph of a cost function against the ratio of the
frequency for two beams shows that the ideal ratio is about 1.27:1,
namely where the cost function is minimised at point. This ratio is
equivalent to the "golden" aspect ratio (i.e., a ratio of about
f02:f20) described in WO97/09842 (U.S. Pat. No. 6,332,029). The
method of improving the modality of a transducer may be extended by
using three piezoelectric beams in the transducer. The ideal ratio
is about 1.315:1.147:1.
[0066] The method of combining active elements, e.g. beams, may be
extended to using piezoelectric discs. Using two discs, the ratio
of sizes of the two discs depends upon how many modes are taken
into consideration. For high order modal density, a ratio of
fundamental frequencies of about 1.1+/-0.02 to 1 may give good
results. For low order modal density (i.e., the first few or first
five modes), a ratio of fundamental frequencies of about 3.2:1 is
good. The first gap comes between the second and third modes of the
larger disc.
[0067] Since there is a large gap between the first and second
radial modes in each disc, much better interleaving is achieved
with three rather than with two discs. When adding a third disc to
the double disc transducer, the obvious first target is to plug the
gap between the second and third modes of the larger disc of the
previous case. However, geometric progression shows that this is
not the only solution. Using fundamental frequencies of f0,
.alpha..f0 and .alpha..sup.2.f0, and plotting
rms(.alpha...alpha..sup.2) there exist two principal optima for a.
The values are about 1.72 and about 2.90, with the latter value
corresponding to the obvious gap-filling method.
[0068] Using fundamental frequencies of f0, .alpha..f0 and
.beta..f0, so that both scalings are free, and using the above
values of .alpha. as seed values, slightly better optima may be
achieved. The parameter pairs (.alpha.,.beta.) are (1.63, 3.03) and
(3.20, 8.19). These optima are quite shallow, meaning that
variations of 10%, or even 20%, in the parameter values are
acceptable.
[0069] An alternative approach for determining the different discs
to be combined is to consider the cost as a function of the ratio
of the radii of the three discs. The cost functions may be RSCD
(ratio of sum of central differences), SRCD (sum of the ratio of
central differences) and SCR (sum of central ratios). For a set of
modal frequencies, f.sub.0, f.sub.1, f.sub.n, . . . f.sub.N, these
functions are defined as:
[0070] RSCD (R Sum CD): 1 RSCD = 1 N - 1 n = 1 N - 1 ( f n + 1 + f
n - 1 - 2 f n ) 2 f 0 SCRD ( sum RCD ) : SRCD = 1 N - 1 n = 1 N - 1
( f n + 1 + f n - 1 - 2 f n f n ) 2 SCR : SCR = 1 N - 1 n = 1 N - 1
( f n + 1 f n - 1 ( f n ) 2 )
[0071] The optimum radii ratio (i.e., where the cost function is
minimised) is 1.3 for all cost functions. Since the square of the
radii ratio is equal to the frequency ratio, for these identical
material and thickness discs, the results of (1.3) (1.3)=1.69 and
the analytical result of 1.67 are in good agreement.
[0072] Alternatively or additionally, passive elements may be
incorporated into the transducer to improve its overall modality.
The active and passive elements may be arranged in a cascade. FIG.
6 shows a multiple disc transducer (70) comprising two active
piezoelectric elements (72) stacked with two passive resonant
elements (74), e.g. thin metal plates so that the modes of the
active and passive elements are interleaved.
[0073] The elements are connected by connecting means in the form
of stubs (78) located at the centre of each active and passive
element. The elements (72, 74) are arranged concentrically. Each
element has different dimensions with the smallest and largest
discs located at the top and bottom of the stack, respectively. The
transducer (70) is mounted on a load device (76), e.g. a panel, by
coupling means in the form of a stub (78) located at the centre of
the first passive device which is the largest disc.
[0074] The method of improving the modality of a transducer may be
extended to a transducer comprising two active elements in the form
of piezoelectric plates. Two plates of dimensions (1 by .alpha.)
and (.alpha. by .alpha..sup.2) are coupled at ({fraction (3/7)},
{fraction (4/9)}). The frequency ratio is therefore about 1.3:1
(1.14.times.1.14=1.2996).
[0075] As shown in FIG. 7, small masses (104) may be mounted at the
end of the piezoelectric transducer (106) having coupling means
(105). In FIG. 8, the transducer (114) is an inertial
electrodynamic moving coil exciter (e.g., as described in
WO97/09842 and U.S. Pat. No. 6,332,029) having a voice coil forming
an active element (115) and a passive resonant element in the form
of a modal plate (118). The active element (115) is mounted on the
modal plate (118) and off-centre of the modal plate.
[0076] The modal plate (118) is mounted on the panel (116) by a
coupler (120). The coupler is aligned with the axis (117) of the
active element (115) but not with the axis (Z) normal to the plane
of the panel (116). Thus the transducer (114) is not coincident
with the panel axis (Z). The active element (115) is connected to
an electrical signal input via electrical wires (122). The modal
plate (118) is perforate to reduce the acoustic radiation therefrom
and the active element (115) is located off-centre of the modal
plate (118), for example, at the optimum mounting position, i.e.
about ({fraction (3/7)}, {fraction (4/9)}).
[0077] FIG. 9 shows a transducer (124) comprising an active
piezoelectric resonant element which is mounted by a coupler (126)
in the form of a stub to a panel (128). Both the transducer (124)
and panel (128) have ratios of width to length of about 1:1.13. The
coupler (126) is not aligned with any axes (130,Z) of the
transducer (124) or the panel (128). Furthermore, the placement of
the coupler (126) is located at the optimum position, i.e.,
off-centre with respect to both the transducer (124) and the panel
(128).
[0078] FIG. 10 shows a transducer (132) in the form of active
piezoelectric resonant element in the form of a beam. The
transducer (132) is coupled to a panel (134) by two couplers in the
form of stubs (136). One stub (136) is located towards an end (138)
of the beam and the other stub (136) is located towards the centre
of the beam.
[0079] FIG. 11 shows a transducer (140) comprising two active
resonant elements (142,143) coupled by a connector (144) and an
enclosure (148) which surrounds the connector (144) and the
resonant elements (142, 143). The transducer (140) is thus made
shock and impact resistant. The enclosure (148) is made of a low
mechanical impedance rubber or comparable polymer so as not to
impede the transducer operation. If the polymer is water resistant,
the transducer (140) may be made waterproof.
[0080] The upper resonant element (142) is larger than the lower
resonant element (143) which is coupled to a panel (145) via a
coupler in the form of a stub (146). The stub (146) is located at
the centre of the lower resonant element (143). The power couplings
(150) for each active element extend from the enclosure (148) to
allow good audio attachment to a load device (not shown).
[0081] FIG. 12 shows a transducer (160) in the form of a plate-like
active resonant element. The resonant element is formed with slots
(162) which define fingers (164) and thus form a multi-resonant
system. The resonant element is mounted on a panel (168) by a
coupler in the form of a stub (166).
[0082] In FIGS. 13A and 13B, the transducer (14) is rectangular
with out-of-plane curvature and is a pre-stressed piezoelectric
transducer of the type disclosed in U.S. Pat. No. 5,632,841
(International patent application WO 96/31333) and produced by PAR
Technologies Inc. under the trade name NASDRIV. Thus, the
transducer (14) is an active resonant element. The transducer has a
width (W) and a length (L) and a position (x) defining an
attachment point (16).
[0083] The curvature of the transducer (14) means that the coupler
(16) is in the form of a line of attachment. When the transducer
(14) is mounted along a line of attachment along the short axis
through the centre, the resonance frequencies of the two arms of
the transducer are coincident. The optimum suspension point may be
modelled and has the line of attachment at about 43% to about 44%
along the length of the resonant element. The cost function (or
measure of "badness") is minimised at this value; this corresponds
to an estimate for the attachment point at {fraction (4/9)}ths of
the length. Furthermore, computer modelling showed this attachment
point to be valid for a range of transducer widths. A second
suspension point at about 33% to about 34% along the length of the
resonant element also appears suitable.
[0084] By plotting a graph of cost (or rms central ratio) against
aspect ratio (AR=W/2L) for a resonant element mounted at 44% along
its length, the optimum aspect ratio may be determined to be about
1.06+/-0.01 to 1 since the cost function is minimised at this
value.
[0085] The optimum angle of attachment .theta. to the panel (12)
may be determined using two "measures of badness" to find the
optimum angle. For example, the standard deviation of the log (dB)
magnitude of the response is a measure of "roughness". Such figures
of merit/badness are discussed in International Application WO
99/41939, and corresponding U.S. patent application Ser. No.
09/246,967, of the present applicants. For an optimised transducer,
namely one with aspect ratio of about 1.06:1 and attachment point
at about 44% using modelling, rotation of the line of attachment
(16) will have a marked effect since the attachment position is not
symmetrical. There is a preference for an angle of about
270.degree., i.e. with the longer end facing left.
[0086] FIGS. 14A and 14B show an asymmetrically shaped transducer
(18) in the form of a resonant element having a trapezium shaped
cross-section. The shape of a trapezium is controlled by two
parameters, AR (aspect ratio) and TR (taper ratio). AR and TR
determine a third parameter, .lambda., such that some constraint is
satisfied, for example, equal mass on either side of the line.
[0087] The constraint equation for equal mass (or equal area) is as
follows: 2 0 ( 1 + 2 TR ( 1 2 - ) ) = 1 ( 1 + 2 TR ( 1 2 - ) )
[0088] The above may readily be solved for either TR or .lambda. as
the dependent variable, to give: 3 TR = 1 - 2 2 ( 1 - ) or = 1 + TR
- 1 + TR 2 2 TR 1 2 - TR 4
[0089] Equivalent expressions are readily obtained for equalising
the moments of inertia, or for minimising the total moment of
inertia.
[0090] The constraint equation for equal moment of inertia (or
equal 2nd moment of area) is as follows: 4 0 ( 1 + 2 TR ( 1 2 - ) )
( - ) 2 = 1 ( 1 + 2 TR ( 1 2 - ) ) ( - ) 2 TR = ( 2 - + 1 ) ( 2 - 1
) 2 4 - 4 3 + 2 - 1 or 1 2 - TR 8
[0091] The constraint equation for minimum total moment of inertia
is: 5 ( 0 1 ( 1 + 2 TR ( 1 2 - ) ) ( - ) 2 ) = 0 TR = 3 - 6 or = 1
2 - TR 6
[0092] A cost function (measure of "badness") was plotted for the
results of 40 FEA runs with AR ranging from 0.9 to 1.25, and TR
ranging from 0.1 to 0.5, with .lambda. constrained for equal mass.
The transducer is thus mounted at the centre of mass. The results
are tabulated below and show that there is an optimum shape with
AR=1 and TR=0.3, giving .lambda. at close to 43%.
1 tr .lambda. 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 0.1 47.51% 2.24%
2.16% 2.16% 2.24% 2.31% 2.19% 2.22% 2.34% 0.2 45.05% 1.59% 1.61%
1.56% 1.57% 1.50% 1.53% 1.66% 1.85% 0.3 42.66% 1.47% 1.30% 1.18%
1.21% 1.23% 1.29% 1.43% 1.59% 0.4 40.37% 1.32% 1.23% 1.24% 1.29%
1.25% 1.29% 1.38% 1.50% 0.5 38.20% 1.48% 1.44% 1.48% 1.54% 1.56%
1.58% 1.60% 1.76%
[0093] One advantage of a trapezoidal transducer is thus that the
transducer may be mounted along a line of attachment which is at
its centre of gravity/mass but is not a line of symmetry. Such a
transducer would thus have the advantages of improved modal
distribution, without being inertially unbalanced. The two methods
of comparison used previously again select about 270.degree. to
about 300.degree. as the optimum angle of orientation.
[0094] The transducer used in the present invention may be seen as
the reciprocal of a distributed mode panel, e.g. as described in
WO97/09842 and U.S. Pat. No. 6,332,029, in that the transducer is
designed to be a distributed mode object.
[0095] It should be understood that this invention has been
described by way of examples only and that a wide variety of
modifications can be made without departing from the scope of the
invention as described in the accompanying claims.
* * * * *